The present invention relates to digital broadcasting systems, and in particular to fast resynchronization of OFDM receivers for digital video broadcasting-handheld (DVB-H) systems.
The DVB-H standard was derived from the DVB-T (DVB Terrestrial) standard. Time slicing is added to allow lower power consumption for handheld devices.
For DVB-H systems, certain scattered pilots (hereinafter “SPs”) are regularly inserted in time- and in frequency-dimensions at a “boosted” power level to the OFDM symbols. The location of scattered pilots is described in clause 4.5.3 of the document ETSI EN 300 744 V1.5.1 (2004-06), which is hereby incorporated by reference.
For the OFDM symbol of index I (ranging from 0 to 67), carriers for which index k (frequency index) belongs to the subset {k=Kmin+3*(I mod 4)+12p|p integer, p>=0. k ε [Kmin; Kmax]} are scattered pilots, wherein p is an integer that takes all possible values greater than or equal to zero, provided that the resulting value of k does not exceed the valid range [Kmin; Kmax]. (See clause 4.5.3 of ETSI EN 300 744 v1.6.1 (2009-01), which is incorporated herein by reference in its entirety).
FIG. 1 shows the frame structure indicating the location of scattered pilots (SPs). In addition to the scattered pilots described above, a frame also includes 177 continual pilots in the 8 K mode and 45 in the 2 K mode (see Table 7 of the ETSI EN 300 744 V1.5.1 (2004-06) document). Clause 4.6 of the ETSI EN 300 744 document also provides the Transmission Parameter Signaling (TPS) information which is transmitted in parallel on 17 TPS carriers for the 2 K mode and on 68 carriers for the 8 K mode. Every TPS carrier in the same symbol conveys the same differentially encoded information bit. Each TPS block, which corresponds to one OFDM frame contains 68 bits, defined as follows:                1 initialization bit;        16 synchronization bits;        37 information bits; and        14 redundant bits for error protection.        
The SP positions are directly related to the OFDM frame. The detection of the frame boundary is the frame synchronization. For example, an initial OFDM symbol window location can be determined by correlating the cyclic prefix. In DVB-T systems, the frame synchronization is performed by correlating for the 16 TPS bits synchronization word. This frame synchronization approach can take 68 ms (one frame) to 136 ms (two frames). In some cases, the receiver needs to find the symbol number (0 to 67) for determining the position of the scattered pilots.
FIG. 2 is a diagram illustrating a conventional synchronization procedure in a DVB-T/H system, where a synchronization sequence must be performed in every time slice. A conventional receiver will need to wake up early to accommodate the following:                1. RF setting and calibrations;        2. AGC settling;        3. Initial window placement and coarse fractional frequency offset estimation;        4. Integer frequency offset estimation;        5. Frame resynchronization;        6. Echo detection and new FFT window placement (if needed);        7. Channel estimation, including fine symbol timing; and        
8. Time drift due to clock error: residual clock ppm error×the time since last reception.
As can be seen in FIG. 2 and described above, a conventional receiver must wake up early to perform the many steps for the frame resynchronization such as enabling the RF frontend, calibrating the I/Q paths, setting the automatic gain control (AGC), correlating a cyclic prefix in order to place an initial OFDM window, estimating fractional and integer frequency offset, and demodulating the TPS carriers to extract the 16 TPS bits synchronization word.
Given the data duration is only around 100 ms in a time slice in DVB-H systems, the synchronization word based resynchronization itself will consume about 50% of the battery capacity. Therefore, it is important that the time for frame resynchronization be reduced.